Hi
You might find it easier just to display a message on the current screen rather than redirecting to another page. In your contact-form.md add in a message section - then this text will display on the contact form page. for example:
otherwise basically you will need to create another “thankyou” folder inside your 01.home folder with a formdata.md file in it for the thank you page. The steps for form setup in pages are here: Example Forms There is a link to the modular form setup from that page.
All that said, probably the most useful is the example theme where you can really see the folders and subfolders in action: Deliver Theme - modular form